Quench and Partitioning Steels

Quench and Partitioning (Q and P) steels are produced by implementing a unique thermal history designed to produce microstructures that contain martensite, and potentially ferrite, along with significant amounts of retained austenite stabilized by high carbon contents. Carbon-stabilized austenite is obtained by carbon transfer from martensite into austenite after a controlled amount of martensite is introduced by judicious selection of a so-called quench temperature at which quenching below the martensite start temperature is interrupted. Following the quench interruption, during the partitioning step, the steel is either held at the quench temperature or brought to a higher temperature and held for a specific time, to stimulate carbon transfer from martensite to austenite, to decrease the carbon supersaturation in martensite and correspondingly stabilize the austenite by carbon enrichment. A final quench to room temperature may be associated with the transformation of a certain fraction of austenite into secondary or “fresh” martensite, which is usually undesirable. A review of the Q and P process is presented, including prediction of austenite retention, alloying effects on its stabilization, a mechanical properties survey, including tensile and local formability relevant to sheet steels for automotive applications, along with perspectives on reactions competing with carbon partitioning that may operate during partitioning.